Acoustic performance of soft elastic media embedded with periodic inclusions

Abstract

Elastic media comprising periodic inclusions are rationally designed composite materials which exhibit acoustic properties that go qualitatively and quantitatively beyond those of their bulk ingredients. A practical application of periodic scatterers in a soft rubber-like medium is in the design of acoustic coatings for maritime vehicles. Acoustic coatings are employed to reduce underwater noise pollution as well as to absorb external sound waves for stealth purposes. The aim of this thesis is to investigate the physical mechanisms governing the performance of underwater acoustic coatings comprising voided and/or hard inclusions embedded in a soft elastic medium. Analytical and numerical models are developed to study the transmission, reflection and absorption properties of the coatings. The analytical models are based on effective medium approximation theory whereby an inhomogeneous medium is modelled as layers of homogeneous media with effective geometric and material properties. The acoustic properties of the layered composites are then obtained using the transfer matrix method. The numerical models are developed using the finite element method whereby the exact geometry and material properties of the acoustic coatings are simulated. The coatings are submerged in water and the effects of water backing and steel-air backing on their acoustic performance are examined. The effects of different material and geometric properties of the coatings on the acoustic performance are also investigated. Different combinations of layers of voids and/or layers of hard inclusions are studied in detail and found to have notably different impacts on the acoustic performance of the coating